The study of hematopoiesis is a prototype for the study of cell differentiation in general. A population of stem cells, maintained throughout life, remains capable of self-renewal, as well as commitment to the lymphoid, myeloid, monocytoid, erythroid, or megakaryocytoid lineages. Numerous investigations are now focussed on identifying the critical genes involved in the decisions governing the differentiation of blood-forming cells. Among the expected features of such genes might be their expression at a pivotal time in hematopoietic development. More intensive attention to such a candidate gene might be merited if it appeared to encode a protein whose features were similar to proteins already implicated as playing roles in other eukaryotic developmental systems.